Process for forming fast-cool vinyl aromatic expandable polymers

ABSTRACT

Expandable vinyl aromatic polymers are produced, which exhibit fast-cool properties upon formation of foamed articles therefrom, by polymerizing a vinyl aromatic monomer, in which is dissolved 1.0-10.0 percent, based on the monomer, of a thermoplastic, branched, block copolymer of 55-95 percent of polymerized vinyl aromatic monomer and 5-45 percent conjugated diene, the polymerization forming vinyl aromatic polymer containing the thermoplastic, branched, block copolymer, and impregnating the polymer so produced, with a blowing agent.

Expandable vinyl aromatic polymer particles, such as polystyrene beads,are commercially prepared using aqueous suspension polymerizationsystems. After formation of the polymer particles, they are normallypre-expanded, such as is described in U.S. Pat. Nos. 3,023,175 and3,577,360 and the pre-expanded particles are then used in the molding offoamed articles.

After the pre-expanded particles are fed to a mold cavity, which definesthe shape of the foamed structure to be produced, the particles areheated above their softening point, such as by steam injected into themold under pressure, and the particles expand to fill the mold cavityand fuse together to form a foamed article. After the particles havebeen treated in the mold to produce a foamed article, the article mustbe cooled for a relatively long time, depending on the size of thearticle, before it can be removed from the mold in a self-supportingstate so as to retain it shape outside the mold. Since such foamedarticles have good insulating properties, the cooling time in the moldconsumes a significant part of the molding cycle and restricts thenumber of foamed articles that can be produced from a mold during agiven time period.

The cooling time is not appreciably reduced by applying cooling media tothe surface of the article or the mold surface since the heat transferthrough the foamed article is extremely slow. Such slow heat transfer isevident by the insulative properties of such foamed articles. If sucharticles are removed from the confines of the mold too soon, thesoftness of the polystyrene and the pressure due to the hot interior ofthe article will cause the article to bulge and lose its desired shape.

Processes have been provided to reduce mold cooling time by addition ofvarious compounds to polystyrene beads during polymerization in aqueoussuspensions, such as are described in U.S. Pat. Nos. 3,389,097 and3,503,908, or coating of polystyrene beads with surface active agents,such as is described in U.S. Pat. No. 3,480,570.

I have found that vinyl aromatic polymer particles in whichthermoplastic, branched, block copolymers have been incorporated bypolymerizing solutions of the vinyl aromatic monomer and thethermoplastic, branched, block copolymer, and which particles have beenimpregnated with a blowing agent, exhibit fast-cool characteristics information of foamed articles therefrom.

BRIEF SUMMARY OF THE INVENTION

Expandable polymer particles, which exhibit fast-cool properties, areproduced by adding to an aqueous suspension system, a vinyl aromaticmonomer in which is dissolved 1.0-10.0 percent by weight, based on themonomer, of a thermoplastic, branched, block copolymer of 55-95 weightpercent of polymerized vinyl aromatic monomer and 5-45 percentconjugated diene, and effecting polymerization to form vinyl aromaticpolymer particles containing said thermoplastic, branched, blockcopolymer, and impregnating the polymer particles with a blowing agentto form expandable vinyl aromatic polymer particles containing thethermoplastic, branched, block copolymer therein.

DESCRIPTION OF THE INVENTION

The present process provides for the production of expandable vinylaromatic polymers having incorporated therein thermoplastic, branched,block copolymers and which exhibit fast-cool properties when molded.

Such vinyl aromatic polymers may be produced from various vinyl aromaticmonomers such as styrene, alpha-methylstyrene, para-methylstyrene,para-t-butylstyrene, mono-chlorostyrene, dichlorostyrene, or mixturesthereof; as well as copolymerization of such vinyl aromatic monomerswith monomers such as divinylbenzene, alkyl and allyl acrylates andmethacrylates, acrylonitrile, maleic anhydride, and the like, whereinthe vinyl aromatic monomer is present in at least 50% by weight of thecopolymer. The most common and preferred vinyl aromatic monomer isstyrene or p-methylstyrene.

In accordance with the present process, the polymerization of the vinylaromatic monomer is effected in a stable aqueous suspension in thepresence of a thermoplastic, branched, block copolymer so as toincorporate the thermoplastic, branched, block copolymer in the polymerparticles produced. The thermoplastic, branched, block copolymer isdissolved in the vinyl aromatic monomer and the solution is suspendedand polymerization effected under conventional polymerizationconditions.

Suspension polymerization techniques are well known. For example, apreferred method employs a suspension system comprised of an inorganicsuspending agent, such as tricalcium phosphate and sodium bisulfitemodifier. Other preferred suspension systems employ organic suspensionstabilizers alone, such as polyvinyl alcohol, hydroxyethyl cellulose,polyvinyl pyrrolidone and the like. Equally useful suspension systemsmay employ combinations of inorganic and organic suspending agents, forexample tricalcium phosphate and polyvinyl alcohol. An importantadvantage of the aqueous suspension systems is that the desired polymerparticles may be obtained as beads of optimum size.

The polymerization may be effected after immediate suspension of themonomer-thermoplastic, branched, block copolymer solution, or themonomer-block copolymer solution may be mass polymerized partially, thensuspended and the polymerization completed in aqueous suspension. Thepolymer products of this invention can, of course, be produced by any ofthe known techniques, such as mass, suspension, or emulsionpolymerization.

As used herein, the term "particles" is meant to cover beads, pellets,coarse grindings, and any other shape having sufficient porosity toallow impregnation without hard cores.

The thermoplastic, branched, block copolymer is present in an amount of1-10 weight percent, based on the weight of the vinyl aromatic monomerand is preferably present in an amount of between 1.0 to 6.0 percent.

The thermoplastic, branched, block copolymer usable in the presentprocess are those block copolymers containing 55-95 percent by weight ofpolymerized vinyl aromatic monomer and 5-45 percent by weight of apolymerized conjugated diene. The vinyl aromatic monomer may be styreneor alpha-methylstyrene. The preferred aromatic monomer is styrene. Theconjugated diene may be butadiene or isoprene.

Such thermoplastic, branched, block copolymers are known compositionsand may be formed, for example, according to the process described inU.S. Pat. No. 3,639,517. Especially useful thermoplastic, branched,block copolymers are available commercially, as for example, KRO-3, anapproximately 75 percent polystyrene, 25 percent butadiene, sold byPhillips Petroleum Company.

The process of the present invention may be used withmonomer-thermoplastic, branched, block copolymer solution to waterratios in the suspension which vary from about 0.3 to 1.5 parts byweight monomer solution per 1.0 part by weight water. The free-radicalinitiating catalysts which are added to the suspension system are theconventional oil-soluble (monomer soluble) catalysts such as organicperoxides, e.g. benzoyl peroxide and t-butyl perbenzoate. Otherfree-radical producing catalysts that may be used in the inventioninclude azobisisobutyronitrile. Conveniently, conventional amounts ofsuch catalysts are used in the invention.

The time and temperature cycles for the polymerization may be thoseconveniently employed. Conveniently, the cycle described in Grim, U.S.Pat. No. 2,673,194, or the two-stage temperature cycle described inD'Alelio, U.S. Pat. No. 2,692,260 is employed. With such a two-stagecycle, in the first stage an elevated temperature of about 50°-90° C. isused for a period of about 3-7 hours, and a second stage usespolymerization temperatures in a range of about 100°-150° C. for aperiod of about 0.25-5.0 hours.

The vinyl aromatic polymer particles, having the thermoplastic,branched, block copolymer incorporated therein, are rendered expandableby impregnating the particles with between 3-12 percent of a volatileblowing agent such as a gas or an agent which will produce a gas onheating. Such blowing agents are preferably one or more compoundsselected from aliphatic hydrocarbons containing from 4 to 6 carbon atomsin the molecule, including butane, pentane, cyclopentane, hexane,cyclohexane, and the halogenated hydrocarbons which boil at atemperature below the softening point of the vinyl aromatic polymer.Such blowing agent, and processes for impregnating the vinyl aromaticpolymer particles therewith, are well known, such as for example isdescribed in U.S. Pat. No. 2,983,692. It is well known to those skilledin the art, that the blowing agent, or mixture of blowing agents, may beincorporated into the polymer particles during the aqueous suspensionpolymerization of monomer solutions.

The modified vinyl aromatic polymer particles, containing a volatileblowing agent, generally in an amount of about 3-12 percent, arepre-expanded by exposing the particles to heat, such as by subjectingthem to steam, hot air or hot water, for example by use of apre-expander as described in U.S. Pat. No. 3,023,175, as is commerciallydone. The pre-expanded particles are then permitted to stand underatmospheric conditions for a period of time, such as two hours to twodays prior to charging the pre-expanded, aged particles to a mold forthe formation of foamed articles.

It will be obvious to those skilled in the art that the present processmay also be used for vinyl aromatic polymer particles that containvarious other additives, such as flame retardant agents, dyes, pigments,anti-static agents, plasticizers, and the like.

The invention is further illustrated by the following examples whereinparts and percentages are by weight unless otherwise indicated.

EXAMPLE I

To a series of 12 oz. crown cap bottles there was charged 100 g water,0.003 g sodium bisulfite, an amount of tricalcium phosphate (TCP) aslisted in Table I, 0.35 g benzoyl peroxide, 0.05 g t-butyl-perbenzoate,and 100 g of a styrene-thermoplastic branched block copolymer solutioncontaining the amount of thermoplastic, branched, block copolymer (TBBC)listed in Table I, which was a styrene-butadiene, branched, blockcopolymer (KRO-3; by Phillips Petroleum Company). The bottles werecapped and suspension polymerization effected by end-over-end agitationof the bottles in a heated oil bath at 90° C. for a 6 hour period,followed by a 2 hour period at 115° C., and cooling to 25° C. over aperiod of 2 hours. The contents of the bottles were emptied andacidified to pH of 1.0 with hydrochloric acid and the removed polymerbeads washed with water. The recovered beads were separated by sievingwith different mesh sieves (U.S. Standard Sieve). A -25+40 bead sizedenotes beads which passed through a 25 mesh sieve and were retained ona 40 mesh sieve. Similarly, a -16+25 beads size denotes the beads whichpassed through a 16 mesh sieve and were retained on a 25 mesh sieve.

Portions of the beads were impregnated with n-pentane by charging to 12oz. bottles, 100 g water containing 0.15 g Triton X-100 (an alkylarylpolyether of octylphenol containing 9-10 ethylene oxide units, Rohm &Haas Co.) as surfactant and 0.5 g tricalcium phosphate, 100 g beads and8.1 g n-pentane. The bottles were capped and heated at 112° C. for 2hours with end-over-end agitation. After cooling to room temperature thecontents were acidified to a pH of about 1.0, centrifuged, filtered andwashed with water, then tray dried at room temperature.

Beads were next pre-expanded in a loosely-capped unstirred five gallonbatch expander by heating for two minutes in steam delivered from a 0.25inch line at a pressure of 12 psig to give densities of approximately1.0 pound per cubic foot (pcf), while higher densities were attained bybleeding air into the steam line to reduce the steam temperature whenconducting the pre-expansion or by shortening the time of contact. Afteraging overnight, the beads were molded into cylindrical moldings 2inches in thickness and 8 inches in diameter. Cool times were determinedand the molded specimens examined for appearance, dimensional stabilityand degree of fusion.

The results are listed in Table I:

                  TABLE I                                                         ______________________________________                                                                        Density of                                                                             Cool                                 Run   TBBC     TCP     Bead     Pre-puff Time                                 No.   (%)      (g)     Size     (pcf)    (sec)                                ______________________________________                                        1     0        0.3     - 16 + 25                                                                              1.0      85                                   2     1        0.3     - 16 + 25                                                                              0.98     85                                   3     2        0.3     - 16 + 25                                                                              1.06     72                                   4     1        0.5     - 25 + 40                                                                              0.98     55                                   5     2        0.5     - 25 + 40                                                                              1.02     38                                   6     2        0.5     - 25 + 40                                                                              0.98     36                                   7     2        0.75    - 35 + 50                                                                              1.10     34                                   ______________________________________                                    

Formed article appearance was good, as was fusion and dimensionalstability. The use of about one percent thermoplastic, branched, blockcopolymer showed only marginal improvement, if any. Smaller bead sizesnormally produce foams which cool faster than foams formed from largerbead sizes.

The beads produced in the suspension polymerization prior toimpregnation with n-pentane were transparent when extruded.

EXAMPLE II

In order to determine cool times of higher density pre-expanded beads, afurther series of bead polymerizations and impregnations (8-13) wereconducted according to the procedure of Example I. The beads utilized inRuns 9 and 12 were made using initiator levels of 0.35 g benzoylperoxide (BPO) and 0.05 g t-butyl perbenzoate (t-BP); while in Runs 10and 13 there was used 0.32 g BPO and 0.05 g t-BP. The polymerizationcycle was 6 hours at 90° C. and 4 hours at 120° C. A commercialpolystyrene was used in Runs 8 and 11. Impregnation of the beads waseffected as in Example I, except that in Runs 11, 12 and 13, 0.16%Triton X-165 (an alkylaryl polyether of octylphenol containing 16ethylene oxide units; Rohm & Haas Co.) was substituted as surfactant.The amount of thermoplastic, branched, block copolymer is designated inTable II which lists the results of cool time tests on beads of a size-25+40:

                  TABLE II                                                        ______________________________________                                                              Density of                                                                              Cool                                          Run     TBBC          Pre-puff  Time                                          No.     (%)           (pcf)     (sec)                                         ______________________________________                                        8       0             1.81      92                                            9       2             1.93      54                                            10      2             2.05      50                                            11      0             1.93      72                                            12      2             1.97      53                                            13      2             2.10      58                                            ______________________________________                                    

Foam appearance and fusion were good in all cases.

EXAMPLE III

Polystyrene -16+30 mesh beads containing 2.6 percent KRO-3 were made ina stirred 100-gallon reactor using a suspension polymerization recipeand cycle comparable to that used in Example I, except that the finalfinishing temperature was 130°-135° C. for two hours. The pre-expandedbeads produced, upon impregnation with n-pentane, showed the following:density of 1.54 pcf gave cool time of 46 sec; density of 1.58 pcf gavecool time of 53 sec. In contrast, straight polystyrene beads of density1.64 pcf gave a cool time of 148 sec. while such beads even of a lowdensity of 1.08 pcf gave a cool time of 81 sec.

EXAMPLE IV

Beads containing 0.25, 0.5, 5.0 and 10.0% KRO-3 were prepared in bottlesvia suspension polymerization similar to previously described examples,using 0.35/0.05 BPO/t-BP initiators and a 90° C./6.0 hour, 135° C./2.0hour polymerization cycle. Impregnation of the above beads along withtwo commercial polystyrene bead samples and the 2.6% KRO-3 beads made inthe 100 gallon reactor with n-pentane was effected at 115° C./2 hours inthe presence of 0.16% Triton X-165. The cool times of the moldedpre-expanded beads are summarized in Table III.

                  TABLE III                                                       ______________________________________                                                                     Density of                                                                            Cool                                     Run     KRO-3   Bead         Pre-puff                                                                              Time                                     No.     (%)     Size         (pcf)   (sec)                                    ______________________________________                                        14      0       - 16 + 30    1.37    93                                       15      0       - 16 + 30    1.43    98                                       16      0.2     - 16 + 30    1.37    88                                       17      0.5     - 16 + 30    1.34    86                                       18      2.6     - 16 + 30    1.28    23                                       19      5.0     - 16 + 30    1.51    41                                       20      10.0    - 16 + 30    1.32    47                                       21      0       - 25 + 40    1.43    76                                       22      5       - 30 + 40    1.55    21                                       23      10      - 30 + 40    1.68    31                                       ______________________________________                                    

It is clear from the results in Table III and the prior results that theincorporation of >1.0% KRO-3 results generally in cool time reductionsof 40-75% as compared to the homo polystyrene controls.

A further advantage of the present process is the fact that the polymerparticles produced containing the thermoplastic, branched, blockcopolymers are transparent when extruded. This is in contrast to vinylaromatic polymer particles containing various elastomers. Thus, ininstances where a bead size of polymer particles resulted, which was notspecifically desirable at the time, the off-size particles or beadscould be readily blended with crystal polystyrenes as a saleableproduct.

What is claimed is:
 1. A process for producing expandable vinyl aromaticpolymer particles which exhibit fast-cooling properties in moldedproducts produced therefrom comprising:polymerizing a vinyl aromaticmonomer in which is dissolved about 1.0 to 10.0 percent by weight, basedon the vinyl aromatic monomer, of a thermoplastic, branched, blockcopolymer of 55-95 weight percent of polymerized vinyl aromatic monomerand 5-45 weight percent of polymerized conjugated diene, to form vinylaromatic polymer particles containing said thermoplastic, branched,block copolymer; and impregnating the vinyl aromatic polymer particles,containing said thermoplastic, branched, block copolymer, with a blowingagent.
 2. The process as defined in claim 1 wherein said thermoplastic,branched, block copolymer is present in an amount of 1.0 to 6.0 percent.3. The process as defined in claim 1 wherein said thermoplastic,branched, block copolymer is formed from a vinyl aromatic monomerselected from the group comprising styrene and alpha-methylstyrene. 4.The process as defined in claim 1 wherein said thermoplastic, branched,block copolymer is formed from a conjugated diene selected from thegroup comprising butadiene and isoprene.
 5. The process as defined inclaim 1 wherein said thermoplastic, branched, block copolymer iscomprised of polystyrene and polybutadiene.
 6. The process as defined inclaim 1 wherein said vinyl aromatic monomer, which is polymerized in thepresence of the thermoplastic, branched, copolymer, is styrene orp-methylstyrene.
 7. The process of claim 1 wherein said vinyl aromaticmonomer-block copolymer solution is polymerized in a stable aqueoussuspension.
 8. The process as defined in claim 1 wherein said blowingagent is selected from the group consisting of aliphatic hydrocarbonshaving 4 to 6 carbon atoms, halogenated hydrocarbons which boil at atemperature below the softening point of the vinyl aromatic polymer, andmixtures thereof.
 9. A process for producing expandable polystyreneparticles which exhibit fast-cooling properties in molded productsproduced therefrom comprising:polymerizing styrene, in which isdissolved about 1.0 to 10.0 percent by weight, based on the styrenemonomer, of a thermoplastic branched block copolymer of 55-95 weightpercent of polystyrene and 5-45 weight percent polybutadiene, to formpolystyrene particles containing said thermoplastic, branched, blockcopolymer; and impregnating the polystyrene particles, containing saidthermoplastic, branched, block copolymer, with a blowing agent.
 10. Theprocess as defined in claim 9 wherein said thermoplastic, branched,block copolymer is present in an amount of 1.0-6.0 percent.